Many mobile devices in use today are based on the Global Standard for Mobile Communications (the GSM standard). The GSM standard is continually evolving and currently consists of a variety of platforms, including the basic GSM standard, General Packet Radio Service (GPRS), Enhanced Data rates for GSM Evolution (EDGE) and third generation GSM services (3GSM) based on the latest Wide Band Code Division Multiple Access (WCDMA) technology. In addition, the 3rd Generation Partnership Project (3GPP) continues to define, refine and enhance the current standards for mobile communications.
GPRS utilizes packet switched data. GPRS includes several mobile networking layers, including, among others, the Radio Resource (RR) sublayer, which provides the functions necessary for RR management of packet data physical channels (PDCHs) and Radio Link Control and Medium Access Control (RLC/MAC) on packet data physical channels. The RR sublayer provides services for the transfer of upper layer PDUs using a shared medium between multiple mobile stations and the network. Direct communication is possible only between the network and one or more mobile stations. The RLC/MAC function supports two modes of operation: unacknowledged operation and acknowledged operation.
The RLC function defines the procedures for segmentation and reassemble of logical link control (LLC) PDUs into RLC/MAC blocks. In RLC acknowledged mode of operation, the RLC function also defines the Backward Error Correction (BEC) procedures enabling the selective retransmission of unsuccessfully delivered RLC/MAC blocks. In RLC acknowledged mode of operation, the RLC function preserves the order of higher layer PDUs provided to it.
The MAC function defines the procedures that enable multiple mobile stations to share a common transmission medium, which may consist of several physical channels. The function may allow a mobile station to use several physical channels in parallel, i.e., to use several timeslots within the time division multiple access (TDMA) frame. For the mobile station originating access, the MAC function provides the procedures, including the contention resolution procedures, for the arbitration between multiple mobile stations simultaneously attempting to access the shared transmission medium. For the mobile station terminating access, the MAC function provides the procedures for queuing and scheduling of access attempts.
Mobile devices based on the aforementioned standards and platforms periodically undergo testing to ensure reliable and effective mobile communications. The 3GPP defines special conformance testing functions that are implemented on mobile devices. In particular, mobile devices based on EDGE technology should support Enhanced GPRS (EGPRS) testing, one test mode of which is referred to as Switch Radio Block Loopback mode.
Switch Radio Block Loopback mode is a physical RF layer loopback performed before channel decoding/encoding and is designed to support Bit Error Ratio (BER) testing. When a mobile device is directed to enter a Switch Radio Block Loopback test mode, the RLC is put into unacknowledged mode and delivers RLC blocks to the physical network layer. When the RLC delivers a quantity of RLC blocks to the physical layer greater than an RLC transmit window size or when the RLC is ready to deliver a block with a countdown (CV) value equal to zero, the RLC should start timer T3182 if it has not received a Packet Uplink ACK/NACK signal. However, if the Switch Radio Block Loopback submode is ON, the RLC cannot receive a Packet Uplink ACK/NACK because control messages are not allowed when this submode is ON. Thus, the RLC starts the T3182 timer, which has a value of five seconds. When the T3182 timer expires, the Uplink Temporary Block Flow (UL TBF) is released and the test mode ends earlier than desired.